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A galactic sky map of sources of interest discussed in Section 3. We have overlaid these sources on the Fermi 4FGL-DR3 sensitivity map for the sake of illustration. The dashed white line represents the minimum observable decl. of the VLA, meaning sources inside the circle will generally have been out of reach of northern observatories. The plot features 21 pulsar candidates, 26 blazar candidates, and 97 empty PSR-like fields. The data providing the locations of the empty MSC fields are available. (The data used to create this figure are available.)
Source publication
Approximately one-third of existing γ -ray sources identified by the Fermi Gamma-Ray Space Telescope are considered to be unassociated, with no known counterpart at other frequencies/wavelengths. These sources have been the subject of intense scrutiny and observational effort during the observatory’s mission lifetime, and here we present a method o...
Contexts in source publication
Context 1
... these γ-ray and radio criteria in hand, we identify 21 sources that we designate as pulsar candidates of high interest, making ideal targets for follow-up. These 21 sources are noted in the MSC and Table 3 with the "psrcandidate" note, as well as illustrated in Figure 4. ...
Context 2
... identify 26 of these blazar-like candidate fields as containing a flat-spectrum radio source, all of which would thus be prime candidates for immediate followup, and notably we are able to do this using only existing catalogs, without the need to further survey these fields. All 26 are illustrated in Figure 4. Twenty of these sources are noted in the MSC and Table 3 with the "blz-candidate" note. ...
Context 3
... identify 537 such fields, a catalog of which is provided among our data products (MSC_empty.fits; see Figure 4). There are several explanations for these sorts of sources, all of which make them somewhat tantalizing targets for followup. ...
Context 4
... an example, the γ-ray properties of these populations might be expected to separate similarly to what was described previously for blazars/pulsars, and so one could conceivably look for superempty fields with pulsar-like γ-ray properties, meeting the criteria established in Section 3.1. We identify 97 of these pulsar-like superempty fields, which we then illustrate in Figure 4. ...
Similar publications
We present the study of multiwavelength observations of an unidentified Fermi Large Area Telescope (LAT) source, 4FGL J1910.7−5320, a new candidate redback millisecond pulsar binary. In the 4FGL 95% error region of 4FGL J1910.7−5320, we find a possible binary with a 8.36 hr orbital period from the Catalina Real-Time Transient Survey, confirmed by o...
Citations
... dio Telescope (GMRT) 150 MHz All-Sky Radio Survey (D. Frail et al. 2016). The various shortcomings of these image-based methods have been discussed by D. A. Frail et al. (2018) but these include low angular resolution (10″), limited fractional bandwidths (<10%), and the low sensitivity of existing synoptic radio surveys (F. K. Schinzel et al. 2017;S. Bruzewski et al. 2023). Fortunately, a new generation of decameter to centimeter radio surveys is underway that represents a substantial improvement in capabilities suited for deeper searches of pulsar candidates (N. Hurley-Walker et al. 2022;P. N. Best et al. 2023; S. W. Duchesne et al. 2023;F. De Gasperin et al. 2023). ...
Over 2000 γ -ray sources identified by the Large Area Telescope on NASA's Fermi Gamma-ray Space Telescope are considered unassociated, meaning that they have no known counterparts in any other frequency regime. We have carried out an image-based search for steep spectrum radio sources, with in-band spectral index < −1.4, within the error regions of Fermi unassociated sources using 1–1.4 GHz radio data from the MeerKAT Absorption Line Survey (MALS) data release. The first MALS data release with a median rms noise of 22–25 μ Jy and 735,649 sources is a significant advance over past image-based searches with improvements in sensitivity, resolution, and bandwidth. Steep spectrum candidates were identified using a combination of in-band spectral indices from MALS and existing radio surveys. We developed an optical and infrared source classification scheme in order to distinguish between Galactic pulsars and radio galaxies. In total, we identify nine pulsar candidates toward six Fermi sources that are worthy of follow-up for pulsation searches. We also report 41 steep spectrum radio galaxy candidates that may be of interest in searches for high-redshift radio galaxies. We show that MALS, due to its excellent continuum sensitivity, can detect 80% of the known pulsar population. This exhibits the promise of identifying exotic pulsar candidates with future image-based surveys with the Square Kilometre Array and its precursors.
... Extensive distributed volunteer community efforts (e.g., Einstein@Home; B. Allen et al. 2013) have been made to bypass this algorithmic difficulty, and many unassociated Fermi-LAT point sources have been blindly followed up using state-ofthe-art radio facilities (e.g., D. A. Frail et al. 2018;S. Bruzewski et al. 2023; C. J. Clark et al. 2023). Other techniques, such as cross-matching Fermi-LAT sources with optical sources exhibiting periodic modulation, have enabled the recovery of several of the timing parameters (e.g., precise position, proper motion, and orbital period), reducing the computational load by more than 8 orders of magnitude and makin ...
The strict periodicity of pulsars is one of the primary ways through which their nature and environment can be studied, and it has also enabled precision tests of general relativity and studies of nanohertz gravitational waves using pulsar timing arrays (PTAs). Identifying such a periodicity from a discrete set of arrival times is a difficult algorithmic problem, In particular when the pulsar is in a binary system. This challenge is especially acute in γ -ray pulsar astronomy, as there are hundreds of unassociated Fermi-LAT sources that may be produced by γ -ray emission from unknown pulsars. Recovering their timing solutions will help reveal their properties and may allow them to be added to PTAs. The same issue arises when attempting to recover a strict periodicity for repeating fast radio bursts (FRBs). Such a detection would be a major breakthrough, providing us with the FRB source’s age, magnetic field, and binary orbit. The problem of recovering a timing solution from sparse time-of-arrival data is currently unsolvable for pulsars in unknown binary systems, and incredibly hard even for isolated pulsars. In this paper, we frame the timing recovery problem as the problem of finding a short vector in a lattice and obtain the solution using off-the-shelf lattice reduction and sieving techniques. As a proof of concept, we solve PSR J0318+0253, a millisecond γ -ray pulsar discovered by FAST in a γ -ray-directed search, in a few CPU minutes. We discuss the assumptions of the standard lattice techniques and quantify their performance and limitations.
... This method has been previously used to identify pulsar candidates from large surveys such as the Giant Metrewave Radio Telescope (GMRT) 150 MHz All-Sky Radio Survey (Frail et al. 2016). The various shortcomings of this image-based methods have been discussed by Frail et al. (2018) but these include low angular resolution ( 10"), limited fractional bandwidths (<10%) and the low sensitivity of existing synoptic radio surveys (Schinzel et al. 2017;Bruzewski et al. 2023). Fortunately, a new generation of decameter to centimeter radio surveys are underway that represents a substantial improvement in capabilities suited for deeper searches of pulsar candidates (Hurley-Walker et al. 2022;Best et al. 2023;Duchesne et al. 2023;de Gasperin et al. 2023). ...
Over 2000 Gamma ray sources identified by the Large Area Telescope (LAT) on NASA's Fermi Gamma-ray Space Telescope are considered unassociated, meaning that they have no known counterparts in any other frequency regime. We have carried out an image-based search for steep spectrum radio sources, with in-band spectral index less than -1.4, within the error regions of Fermi unassociated sources using 1 to 1.4 GHz radio data from the MeerKAT Absorption Line Survey (MALS) Data Release. MALS DR1 with a median rms noise of 22 to 25 microJy and 735,649 sources is a significant advance over past image-based searches with improvements in sensitivity, resolution and bandwidth. Steep spectrum candidates were identified using a combination of in-band spectral indices from MALS and existing radio surveys. We developed an optical and infrared source classification scheme in order to distinguish between galactic pulsars and radio galaxies. In total, we identify nine pulsar candidates towards six Fermi sources that are worthy of follow-up for pulsation searches. We also report 41 steep spectrum radio galaxy candidates that may be of interest in searches for high-redshift radio galaxies. We show that MALS due to its excellent continuum sensitivity can detect 80 percent of the known pulsar population. This exhibits the promise of identifying exotic pulsar candidates with future image-based surveys with the Square Kilometre and its precursors.
... Previous image-based candidate searches have been plagued by contaminants (variables, extragalactic sources, etc.; e.g., C. De Breuck et al. 2000; D. L. Kaplan et al. 2000;F. de Gasperin et al. 2018;Y. Maan et al. 2018;S. Bruzewski et al. 2023). This current work is a substantial improvement over past image-based pulsar searches in several dimensions: sensitivity, improved angular resolution/astrometry, polarization, in-band spectra, and the use of multiple selection criteria. The rms noise levels of these mosaic images are 12-17 μJy ba −1 (see Table 1). When past continuum su ...
We report on the results of an image-based search for pulsar candidates toward the Galactic bulge. We used mosaic images from the MeerKAT radio telescope that were taken as part of a 173 deg ² survey of the bulge and Galactic center of our Galaxy at L band (856–1712 MHz) in all four Stokes I , Q , U , and V . The image rms noise levels of 12–17 μ Jy ba ⁻¹ represent a significant increase in sensitivity over past image-based pulsar searches. Our primary search criterion was circular polarization, but we used other criteria, including linear polarization, in-band spectral index, compactness, variability, and multiwavelength counterparts to select pulsar candidates. We first demonstrate the efficacy of this technique by searching for polarized emission from known pulsars and comparing our results with measurements from the literature. Our search resulted in a sample of 75 polarized sources. Bright stars or young stellar objects were associated with 28 of these sources, including a small sample of highly polarized dwarf stars with pulsar-like steep spectra. Comparing the properties of this sample with the known pulsars, we identified 30 compelling candidates for pulsation follow-up, including two sources with both strong circular and linear polarization. The remaining 17 sources are either pulsars or stars, but we cannot rule out an extragalactic origin or image artifacts among the brighter, flat-spectrum objects.
... If the IR association is included, the classification changes to a confident AGN (CT = 8.8). The detection of a radio source, MSC J204116.66+473659.2, with a flux of 0.65 ± 0.12 mJy at 5.9 GHz and spectral index α = 0.35 ± 0.25 (Bruzewski et al. 2023), coincident with the X-ray source, supports the AGN classification for CXO J204116.7 +473658. The identification of the only X-ray counterpart as an AGN suggests that the 4FGL source may be an AGN. ...
In the latest data release from the Fermi Gamma-ray Space Telescope (the 4th Fermi LAT 14 yr Catalog, or 4FGL), more than 50% of the Galactic sources are yet to be identified. We observed 13 unidentified Fermi LAT sources with the Chandra X-Ray Observatory to explore their nature. We report the results of the classification of X-ray sources in the fields of these γ -ray sources and discuss the implications for their nature. We use multiwavelength (MW) data for a machine-learning classification, accompanied by a more detailed spectral/variability analysis for brighter sources. Eight 4FGL sources have γ -ray pulsars within their position error ellipses. We consider three of these pulsars (PSR J1906+0722, PSR J1105–6037, and PSR J1358–6025) to be detected in X-rays, while PSR J1203–6242 shows a hint of X-ray emission. Within the positional uncertainties of three of the 4FGL sources, we detect X-ray sources that may be yet unknown pulsars, depending on the MW association. In addition to point sources, we discovered two extended sources, one of which is likely to be a bow-shock pulsar-wind nebula associated with PSR J1358–6025. Finally, we classify other X-ray sources detected in these observations and report the most interesting classifications.
... If the IR association is included, the classification changes to a confident AGN (CT=8.8). The detection of a radio source, MSC J204116.66+473659.2, with a flux of 0.65 ± 0.12 mJy at 5.9 GHz and spectral index α = 0.35 ± 0.25 (Bruzewski et al. 2023), coincident with the X-ray source, supports the AGN classification for CXO J204116.7+473658. The identification of the only X-ray counterpart as an AGN suggests that the 4FGL source may be an AGN. ...
In the latest data release from the Fermi -Ray Space Telescope (the 4th Fermi LAT 12-year Catalog or 4FGL) more than 50% of the Galactic sources are yet to be identified. We observed thirteen unidentified Fermi LAT sources with Chandra X-Ray Observatory (CXO) to explore their nature. We report the results of the classification of X-ray sources in the fields of these -ray sources and discuss the implications for their nature. We use the multiwavelength (MW) data for machine-learning classification accompanied by a more detailed spectral/variability analysis for brighter sources. Seven 4FGL sources have -ray pulsars within their position error ellipses. Three of these pulsars are either detected in the CXO images or show hints of X-ray emission. Within the positional uncertainties of three 4FGL sources we detect X-ray sources that may be yet unknown pulsars, depending on the MW association. In addition to point sources, we discovered 2 extended sources one of which is likely to be a bowshock pulsar-wind nebula associated with PSR J1358.3-6026. Finally, we classify other X-ray sources detected in these observations and report most interesting classifications.
Context . The latest source catalog of the Fermi- LAT telescope contains more than 7000 γ -ray sources at giga-electronvolt energies, with the two dominant source classes thought to be blazars and rotation-powered pulsars. Despite continuous follow-up efforts, around 2600 sources have no known multiwavelength association.
Aims . Our target is the identification of possible (young and recycled) pulsar candidates in the sample of unassociated γ -ray sources via their characteristic X-ray and γ -ray emission. To achieve this, we cross-matched the Fermi -LAT catalog with the catalog of X-ray sources in the western Galactic hemisphere from the first four all-sky surveys of eROSITA on the Spektrum-Roentgen-Gamma (SRG) mission. We complement this by identifying X-ray counterparts of known pulsars detected at γ -ray and radio energies in the eROSITA data.
Methods . We used a Bayesian cross-matching scheme to construct a probabilistic catalog of possible pulsar-type X-ray counterparts to Fermi -LAT sources. Our method combines the overlap of X-ray and γ -ray source positions with a probabilistic classification (into pulsar and blazar candidates) of each source based on its γ -ray properties and a prediction on the X-ray flux of pulsar- or blazar-type counterparts. Finally, an optical and infrared counterpart search was performed to exclude coronally emitting stars and active galactic nuclei from our catalog.
Results . We provide a catalog of our prior γ -ray-based classifications of all 2600 unassociated sources in the Fermi -LAT catalog, with around equal numbers of pulsar and blazar candidates. Our final list of candidate X-ray counterparts to suspected new high-energy pulsars, cleaned for spurious detections and sources with obvious non-pulsar counterparts, contains around 900 X-ray sources, the vast majority of which lie in the 95% γ -ray error ellipse. We predict between 30 and 40 new pulsars among our top 200 candidates, with around equal numbers of young and recycled pulsars. This candidate list may serve as input to future follow-up campaigns, looking directly for pulsations or for the orbital modulation of possible binary companions, where it may allow for a drastic reduction in the number of candidate locations to search. We furthermore detect the X-ray counterparts of 15 known rotation-powered pulsars, which were not seen in X-rays before.
